Stereotactic injections of horseradish peroxidase have been made into the monkey optic nerve in order to map the location of retrogradely transported HRP in ganglion cells. The arcuate nerve fiber region, corresponding to that most susceptible to glaucomatous damage in man, has been mapped. Ultrastructural studies will be directed toward clarifying morphologic changes in these areas in animals subjected to increased intraocular pressure experimentally.